The present invention relates generally to polymeric binders used in photoconductors. More particularly, this invention relates to low temperature cure polyvinylbutyral and a process for its synthesis.
Electrophotography is a process in which image information in the form of electronic signals is converted to optical signals which are then converted to a latent electrostatic field and stored until developed into a visible image and transferred to paper. Commercial applications of electrophotography include photocopiers and non-impact (i.e., electronic) printers. See, generally, J. Mort, The Anatomy of Xerography (McFarland and Co., 1989); J. L. Johnson, Principles of Non Impact Printing (Palatino Press, 1986).
The conversion of an optical image pattern to a latent electrostatic charge pattern is the function of the photoreceptor. The photoreceptor is a photoconductive insulator having charge generating and charge transport properties. In the process of electrophotography, the surface of the photoreceptor is charged with ions in the dark, then selectively discharged by exposure to an optical image. The degree of discharge is proportional to the light intensity in the optical image provided that the photogenerated charge carriers are efficiently transported away from the exposed surface (i.e., the required carrier mobility must be at least 10xe2x88x926 cm2/V second). The latent image thus produced is subsequently developed and fixed into a permanent image.
Many photoreceptors can be used both in photocopiers and in non impact printing. Although speed, spectral sensitivity and durability requirements differ for the two applications, the main requirements of a photoreceptor are: high photosensitivity to visible light, good carrier transport properties, and high dark resistivity. Uniform and low-cost manufacturing and the ability to tailor a photoreceptor""s spectral responses to particular applications are also advantageous.
For these reasons, photoconductors comprised of dye molecules dispersed in an electrically inert cross-linked polymer are widely used commercially. Photoexcitation induces electronic transitions in dye molecules and initiates a cascade of intermolecular charge transport. The efficiency of this process, which involves the hopping of holes or electrons from one dye molecule to another, requires the presence of high concentrations of dye (circa 1020 molecules per cc) randomly dispersed in the polymer.
Various polymers have been used as pigment dispersing binders in charge generation layers, e.g., polyester, phenoxy resins, poly(vinyl acetate), and, most recently, poly(vinyl butyral) (see Proceedings, Sixth International Congress on Advances in Non-Impact Printing Technologies, Orlando, Fla., 1990, pp. 312-346). Poly(vinyl butyral) (PVB) is particularly preferred for its solubility in a wide range of solvents, its ability to coat many kinds of organic pigments thus providing stable dispersion throughout the charge generation layer, and its excellent film-forming properties. Conventional PVB has the structure, 
where x=60 to 95 mol %, y=3 to 40 mol %, and z=0 to 20 mol %, and x+y+z=100%.
The only reactive functional group for cross-linking the polymer is the hydroxy group of the polyvinyl alcohol component. Because of the relatively low reactivity of this group, elevated temperatures and long reaction times or strong acid or base catalysts are required. These conditions may adversely affect the performance of the photoreceptor by altering the stability of pigment and charge transport molecules contained therein. Accordingly, there is a need for poly(vinyl butyral) that can be cross-linked readily without resorting to harsh conditions of temperature and pH and, correspondingly, for a convenient and efficient method of synthesizing such a compound.
The invention provides new polymeric compounds which are useful as binders in photoconductors as well as in surface coating materials, structural adhesives, and other applications where low temperature cure may be advantageous or required.
Accordingly, one aspect of the invention is directed to compounds of Formula I: 
wherein:
R1 is alkyl, dialkyl ether, alkyl carbonyl, cycloalkyl, cycloalkyl alkyl, aryl, aryl carbonyl, alkaryl carbonyl, aralkyl, or alkyl aralkyl;
Z is xe2x80x94OH or xe2x80x94NHR2, where R2 is independently alkyl, dialkyl ether, alkyl carbonyl, cycloalkyl, cycloalkyl alkyl, aryl, aryl carbonyl, alkaryl carbonyl, aralkyl, alkyl aralkyl, or R2 is H.
x is 60 to 95 mol %, y is 3 to 40 mol %, z is 0 to 20 mol %; and
x+y+z is 100 mol %.
Another aspect of the invention provides a process for preparing a compound of Formula I, where Z is xe2x80x94OH, by hydrolysing a compound of Formula (3) having the structure: 
with an acid, where R1, x, y, z are as defined above.
Yet another aspect of the invention is directed to an intermediate in the synthesis of a compound of Formula I, wherein the intermediate is a compound of Formula (3).
Another aspect of the invention is to provide a photoconductor for electrophotography, wherein the photoconductor comprises a charge generation layer and a charge transport layer, the charge generation layer is formed from a photoconductive pigment dispersed in a binder, and the binder is comprised of a poly(vinyl butyral) compound of Formula I.
1. Definitions
The following definitions are set forth to illustrate and define the meaning and scope of the various terms used to describe the invention herein.
xe2x80x9ct-BOCxe2x80x9d means t-butoxycarbonyl.
xe2x80x9cCGLxe2x80x9d means charge generation layer.
xe2x80x9cPVBxe2x80x9d means poly(vinyl butyral).
As used herein, the term xe2x80x9calkylxe2x80x9d means a linear or branched chain mono- or di-valent radical consisting solely of carbon and hydrogen, containing no unsaturation and having from one to 10 carbon atoms, e.g., methyl, methylene, propyl, trimethylene, heptyl and the like, which can be optionally substituted with halogen or aryl groups.
As used herein, the term xe2x80x9cdialkyl etherxe2x80x9d refers to a mono- or di-valent radical of the formula xe2x80x94Raxe2x80x94Oxe2x80x94Rb, where Ra and Rb can be the same or different alkyl groups, and Rb in a divalent radical forms a bond with Z of Formula I, e.g. diethyl ether,oxydiethylene, methoxyethylene, and the like.
As used herein, the term xe2x80x9calkyl carbonylxe2x80x9d refers to a mono- or di-valent radical of the formula xe2x80x94C(O)Ra where Ra is alkyl as defined above, and Ra in a divalent radical forms a bond with Z of Formula I, e.g., methyl carbonyl, carbonyl ethylene, and the like.
As used herein, the term xe2x80x9ccycloalkylxe2x80x9d refers to a mono- or di-valent ring radical consisting solely of carbon and hydrogen atoms, containing no unsaturation and having from five to seven carbon atoms, e.g,. cyclopentyl, cyclohexylene, and the like.
As used herein, the term xe2x80x9ccycloalkyl alkylxe2x80x9d refers to a mono- or di-valent radical of the formula xe2x80x94Raxe2x80x94Rb where Ra is alkyl and Rb is cycloalkyl, and Rb in a divalent radical forms a bond with Z of Formula I, e.g., cyclopentyl methylene, cyclohexylene ethylene, and the like.
As used herein, the term xe2x80x9carylxe2x80x9d refers to a mono- or di-valent unsaturated aromatic carbocyclic radical having one or more rings, e.g., phenyl, naphthyl, phenylene, and the like, which can be optionally substituted with halogen or aryl groups.
As used herein, the term xe2x80x9caryl carbonylxe2x80x9d refers to a mono- or di-valent radical of the formula xe2x80x94C(O)Rc where Rc is aryl, and Rc in a divalent radical forms a bond with Z of Formula I, e.g., phenylcarbonyl, carbonyl phenylene, and the like.
As used herein, the term xe2x80x9calkaryl carbonylxe2x80x9d refers to a mono- or di-valent radical of the formula xe2x80x94Rcxe2x80x94Ra where Rc is aryl and Ra is alkyl, and Ra in a divalent radical forms a bond with Z of Formula I, e.g., methylphenyl carbonyl, methylene phenylcarbonyl, and the like.
As used herein, the term xe2x80x9caralkylxe2x80x9d refers to a mono- or di-valent radical of the formula xe2x80x94Raxe2x80x94Rc where Ra is alkyl and Rc is aryl, and Rc in a divalent radical forms a bond with Z of Formula I, e.g., benzyl, methylene phenylene, and the like.
As used herein, the term xe2x80x9calkyl aralkylxe2x80x9d refers to a mono- or di-valent radical of the formula xe2x80x94Rcxe2x80x94Ra, where Rc is aralkyl as defined above and Ra is alkyl, and Ra in a divalent radical forms a bond with Z of Formula I, e.g. methyl benzyl, methylene phenylethylene, and the like.
As used herein, the term xe2x80x9chydrogen halide scavengerxe2x80x9d means a compound capable of removing hydrogen halide from a reaction mixture, e.g., potassium carbonate, sodium carbonate, potassium bicarbonate, sodium bicarbonate, triethylamine, tributylamine, N,Nxe2x80x2-dialkylamine, and the like
The term xe2x80x9chydrolyzingxe2x80x9d or xe2x80x9chydrolysisxe2x80x9d refers to the process of splitting a chemical bond by the addition of water; for example, hydrolysis of an ether gives an alcohol, hydrolysis of an alkyl ester gives an organic acid and an alcohol. Hydrolysis may be accomplished by treatment with an inorganic or organic acid, or by treatment with a base.
xe2x80x9cOptionalxe2x80x9d or xe2x80x9coptionallyxe2x80x9d means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances in which it does not.
2. Utility
The compounds of Formula I are useful as binders in photoconductors as well as in surface coating materials, structural adhesives, and other applications where low temperature cure may be advantageous or required. The compounds of Formula I exhibit excellent adhesion to a wide variety of surfaces. When used as pigment dispersing binders in charge generating layers of photoconductors, the compounds of Formula I exhibit higher photoresponses than do comparable CGLs comprising conventional PVB. Because of their ability to undergo cross-linking at lower temperatures than conventional PVB, these compounds can be used with charge transport molecules and pigments that are sensitive to degradation by high temperatures and/or acid catalysts that are typically used during cross-linking of conventional PVB. The conditions required for cross-linking the compounds of Formula I are safer, less costly and more environmentally favorable than are conditions required for cross-linking conventional PVB.
3. Preferred Embodiments
Preferred are the compounds of Formula I where Z is xe2x80x94OH and R1 is alkyl, dialkyl ether, alkyl carbonyl, cycloalkyl, cycloalkyl alkyl, aryl carbonyl, alkaryl carbonyl, aralkyl, or alkyl aralkyl. Particularly preferred are those compounds where R1 is xe2x80x94CH2xe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CH2CH2OCH2CH2xe2x80x94, xe2x80x94C(O)CH2xe2x80x94, xe2x80x94C(O)CH2CH2xe2x80x94, and xe2x80x94CH2C6H10, and x is about 78% to about 82 mol %, y is about 15% to about 17 mol % and z is about 1% to about 7 mol %.
Also preferred are the compounds of Formula I where Z is xe2x80x94NHR2, where R2 is alkyl, dialkyl ether, alkyl carbonyl, cycloalkyl, cycloalkyl alkyl, aryl, aryl carbonyl, alkaryl carbonyl, aralkyl, alkyl aralkyl, or R2 is H, and R1 is the same as for compounds of Formula I where Z is xe2x80x94OH. Particularly preferred are those compounds where R1 is xe2x80x94CH2xe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CH2CH2OCH2CH2xe2x80x94, xe2x80x94C(O)CH2xe2x80x94, xe2x80x94C(O)CH2CH2xe2x80x94, and xe2x80x94CH2C6H10xe2x80x94, R2 is H, and x is about 78 mol % to about 82 mol %, y is about 15 mol % to about 17 mol % and z is about 1 mol % to about 7 mol %.
4. Synthesis of Compounds of Formula I
A. Synthetic Reaction Parameters
The terms xe2x80x9csolventxe2x80x9d, xe2x80x9cinert organic solventxe2x80x9d or xe2x80x9cinert solventxe2x80x9d mean a solvent inert under the conditions of the reaction being described in conjunction herewith (including, for example, benzene, dichloromethane, tetrahydrofuran, dimethylformamide, acetonitrile, and the like, but excluding alcoholic solvents such as methanol, ethanol, isopropanol, and the like). Unless specified to the contrary, the solvents used in the reactions of the present invention are inert organic solvents.
Unless specified to the contrary, the reactions described herein take place at atmospheric pressure within a temperature range from 10xc2x0 C. to 100xc2x0 C., preferably from 15xc2x0 C. to 90xc2x0 C.; most preferably from xe2x80x9croomxe2x80x9d or xe2x80x9cambientxe2x80x9d temperature, e.g., 20xc2x0 C., to 80xc2x0 C. Further, unless otherwise specified, the reaction times and conditions are intended to be approximate, e.g., taking place at about atmospheric pressure within a temperature range of about 5xc2x0 C. to about 100xc2x0 C., preferably from about 15xc2x0 C. to about 90xc2x0 C., most preferably from about 20xc2x0 C. to about 90xc2x0 C. over a period of about 14 to about 30 hours. Parameters given in the Examples are intended to be specific, not approximate.
Isolation and purification of the compounds and intermediates described herein can be effected, if desired, by any suitable separation or purification procedure such as, for example, filtration, extraction, centrifugation, chromatography, or a combination of these procedures. Specific illustrations of suitable separation and isolation procedures can be had by reference to the examples hereinbelow. However, other equivalent separation or isolation procedures can, of course, also be used.
B. Preparation of Compounds of Formula (I), Where Z is xe2x80x94OH
The process for the preparation of the compounds of Formula I, where Z is xe2x80x94OH, is shown below in Reaction Scheme 1, in which the substituent group R1 and x, y and z have the same meanings as described in the Summary of the Invention, unless otherwise specified. 
Starting Materials
Polyvinyl butyral (PVB, Formula (1)), R1Br-substituted alcohol and dihydropyran are commercially available. The tetrahydropyran compound of Formula (2) is formed by reacting R1Br-substituted alcohol with dihydropyran by methods known in the art.
Formula (3)
As illustrated in Reaction Scheme 1, Step 1, PVB (1) is reacted with a compound of Formula (2) to give a corresponding compound of Formula (3).
The reaction is carried out in a nitrogen atmosphere in the presence of excess hydrogen halide scavenger, most preferably potassium carbonate. The reactants are heated in an inert organic solvent at about 70xc2x0 C. to 90xc2x0 C. to yield a compound of Formula (3). When the reaction is substantially complete, typically at 24 hours, the compound of Formula (3) is isolated from the reaction mixture by standard techniques, for example, by precipitation and filtration.
Formula (I), Z is xe2x80x94OH
As illustrated in Reaction Scheme 1, Step 2, the tetrahydropyran group is selectively removed from a compound of Formula (3) to yield the compound of Formula (I), where Z is xe2x80x94OH.
Selective removal of the tetrahydropyran group is achieved by dissolving the product of Step 1 in an inert solvent, adding to it dropwise a catalytic amount of a dilute acid, such as 10% HCl, and incubating the reaction mixture at room temperature until deprotection is complete, normally about 18 hours. The compound of Formula I is isolated from the reaction mixture, purified and concentrated by standard techniques, for example, by precipitation, and evaporation of volatile compounds under reduced pressure.
C. Preparation of Compounds of Formula (I), Where Z is xe2x80x94NHR2 
The process for the preparation of the compounds of Formula I, where Z is xe2x80x94NHR2, is shown below in Reaction Scheme 2, in which the substituent group R2 and x, y and z have the same meanings as described in the Summary of the Invention, unless otherwise specified. 
Formula (4)
Compounds of formula (4) are intermediates in the preparation of compounds of Formula I, Z=xe2x80x94NHR2 as illustrated in Reaction Scheme 2. The compounds of Formula (4) are prepared by reacting brominated mono- or di-substituted amine or a corresponding salt thereof (commercially available) with di-t-butyldicarbonate according to methods known to those of ordinary skill in the art. Other amino protecting groups that can be removed easily with mild acids can also be used (e.g., amyl carbamate).
Formula (5)
A compound of Formula (5) is prepared by reacting PVB (Formula (1)) with a compound of Formula (4) in the presence of base using the same conditions as described for the preparation of Formula (3) above. When the reaction is substantially complete, the compound of Formula (5) is purified by precipitation.
Formula (I), Z is xe2x80x94NHR2 
The amino-protecting group of a compound of Formula (5) is hydrolyzed in the presence of mild acid, e.g., trifluoroacetic acid, or alternatively, is removed by catalytic hydrogenation, to yield the compound of Formula I, where Z is xe2x80x94NHR2.